The present invention relates to a radiation detector for detecting radiation such as X-rays and .gamma.-rays, and more particularly to a radiation detector suitable for use in an X-ray CT (computed tomography) or positron camera.
Various types of X-ray CT have been known. In a typical example thereof, a turntable member having an aperture at a central portion thereof is arranged vertically, and a body or target to be examined is placed at the center of the aperture provided in the turntable member. An X-ray source is mounted on a circumferential part of the turntable member. The X-rays emitted from the X-ray source travel fanwise, pass through the target to be examined, and are then detected by a radiation detecting means mounted on another circumferential part of the turntable member. The radiation detecting means includes an array of thirty to one-thousand radiation detectors which have the same performance and are arranged on an arc of a circle with center at the X-ray source. In the case where narrow or limited X-ray sector span is used, the detectors may be arranged on a straight line. The X-ray source and the radiation detecting means mounted on the turntable member are rotated about the target by rotating the turntable member about the center axis of the aperture, and the output of each radiation detector is measured every time the turntable member is rotated by a predetermined angle (for example, 1.degree.). On the basis of the measured values, a cross sectional image of the target to be examined is reconstructed.
A gas chamber filled with Xenon or the combination of bismuth germanate (BGO) monocrystals and photomultipiers has hiterto been used as the radiation detecting means for the X-ray CT or the like. In these detecting means, it was not easy to make the respective characteristics of channels the same. Therefore, it was difficult to obtain an acceptable clear image. Specifically, in the combination of BGO monocrystals and photomultipliers, the fluctuation in the characteristics of the BGO monocrystals as a scintillator as well as the fluctuation in the characteristics of the photomultipliers resulted in a great difficulty of the provision of radiation detectors having the same characteristics.
In order to solve this problem, some of the present inventors have proposed a radiation detector using phosphor particles as a scintillator, in the Japanese Patent Application No. 77887/78 (corresponding to the U.S. Application Ser. No. 47133 filed on June 11, 1979 and the German Patent Application No. P2923324.7) on June 8, 1979. A radiation detector for X-ray CT usually has a width of 1 to 10 mm (preferably 1 to 3 mm) and a length of, for example, about 20 mm in order to obtain a high-accuracy sectional image. Accordingly, the number of phosphor particles included in one radiation detector is, for example, on the order of 300,000, though depending upon the particle size. The respective characteristics of phosphor particles may be slightly different. However, when the phosphor particles are used as one scintillator after sufficient mixing thereof, fluctuations in the characteristic of the scintillator are on the order of the reciprocal of the square root of the number of phosphor particles, that is, on the order of 0.01 percent, and therefore satisfactory results can be obtained.
An efficiently available light emission output generated through radiation stimulation from the phosphor particle scintillator includes light which is generated in a surface region of the scintillator and light which is generated at the inner parts of the scintillator and escape to the outside of the scintillator. The escape of light is difficult due to scattering in the scintillator. This internal scattering results in the substantial absorption of light energy, thereby lowering an externally available light output from the scincillator. Thus, it is desirable to use a phosphor material which has a high radiation absorption power and a high radiation-light conversion efficiency. Further, it is desirable to use a tilted or multi-layer scintillator in order to facilitate the escape of light.
However, the present inventors' experiments have revealed that a phosphor particle scintillator encounters a great fluctuation (hereinafter referred to as noise) in the outputs from the same scintillator over a number of measurements. Thus, even if the radiation-light conversion efficiency of a phosphor material or the degree of escape of internally generated light is enchanced to improve the light output from a scintillator, it was difficult to sufficiently improve the signal-to-noise (S/N) ratio.